The location of a mobile station can be determined by a positioning procedure, for example a positioning procedure which is part of a location service.
Location services in cellular systems are based on determining the delay of signals transmitted by different network elements of a cellular network to a mobile station of which the location is to be determined. In case of a line-of-sight transmission, the delay of the signals is directly dependent on the distance between the mobile station and the respective transmitting network element.
A location service puts other demands on the acquisition of signals than usual communication services, and in particular on the finger searching process (a process known for RAKE type receivers) for the delay estimation using information of an impulse response.
For usual communication services, the strongest signals are of the highest interest. The searching can therefore be carried out by picking up the peaks in the impulse responses. For location services, in contrast, it is an aim to find the signal which arrives first, preferably the line of sight signal, if available.
For usual communication services, further the covering range is different, since only the signals from the serving network element have to be acquired. The serving network element is the network element serving the cell in which the mobile terminal is currently located. Thus for usual communication services, the delay profile is mainly determined by the cell size of the network. For location services, in contrast, the covering range is determined by the size of the server cell and the neighboring cells, since signals from a plurality of network elements are required for determining the exact position of the mobile station.
For illustration, FIG. 1 shows a cellular network with a server base station BSs as serving network element in a server cell 10 having a hexagonal shape. The mobile station MS, of which the current location is to be determined, is situated in this server cell 10. The server cell 10 is surrounded by six further hexagonal cells 11, which are served by immediately neighboring base stations BSn constituting further network elements. Additional base stations, which are still farther away from the mobile station MS, might have to be taken into account in addition.
The searching procedure for detecting a delayed signal for a location service is normally carried out in the impulse response of signals received from different network elements. The searching may be performed with a correlation procedure, e.g. with a matched filter, starting from a zero delay. The delay is then increased until a correspondence is found. The delay can be estimated e.g. by an edge detection in the impulse response profile of the received signals. The length of the impulse response profile is much longer than the width of the signal shape. Therefore, the search for an edge is started from a certain position of the signal by comparing the amplitude of sampling data with a pre-defined threshold. On the one hand, this threshold has to be set high enough in order to avoid that a noise peak is detected as the signal edge, which would result in a false alarm. On the other hand, the threshold has to be set low enough to guarantee that the signal edge is detected even if the signal strength is rather weak.
Principally, the length of the impulse response should be long enough to cover the possible maximum delay. It is known from the searching procedures for delayed signals, however, that the length of the searching range generally affects the false-alarm rate in the acquisition of a signal. The longer the searching range, the higher the false alarm rate will be. The problem is even more severe for location services, since the signals are partly much weaker and the noise is higher for neighboring network elements, while the delays are bigger. The majority of the noise is the interference caused by the server network element when acquiring the neighboring network element.
The same kind of problems may occur in other cellular network based positioning procedures.